The subject matter of this document relates to optical communications, including apparatus, methods, systems and applications of such apparatus, methods and systems.
Optical communications use an optical modulator to modulate an optical carrier beam to carry digital bits for transmission over an optical link. Optical carriers have broad optical bandwidths that are much greater than the bandwidths of RF and microwave carriers and can use optical wavelength division multiplexing (WDM) to use a single fiber or optical waveguide to transmit multiple optical carriers modulated to carry different optical data channels.
Various optical modulation techniques have been developed for modulating an optical carrier to carry digital data. For example, the binary phase shift keyed (BPSK) modulation modulates an optical carrier to produce different optical power levels to represent logical levels of “0” and “1” with phase shifts of 0 and π radians. The phase shifted optical pulses for logical “0” and “1” are decoded at an optical receiver by determining whether the detected signal is to the left or right of a vertical imaginary axis to represent the quadrature phase (Q) component in a signal vector diagram in which the horizontal real axis represents the in-phase (I) component. In some applications, a phase detector can be used to detect the value of the received phase and to determine the logical level of an optical pulse, where a phase value greater than π/2 corresponds to the logical “0” and a phase value less than π/2 corresponds to the logical “1.” In other applications, the cosine of the phase of the signal can also be detected to determine the logical level of an optical pulse, where a negative cosine value can be used to represent the logical level “0” and a positive cosine value can be used to represent the logical level “1.” The BPSK optical signals may use a differentially-encoded phase shift keyed (DeBPSK, or DPSK) modulation format. The DPSK modulation format encodes input data as the difference between two consecutive transmitted symbols. The input data is differentially pre-coded using the preceding symbol as a reference with an electrical “delay+add” function so that an input data bit of logical “0” or “1” is encoded as a change of carrier phase of 0 or π radians relative to the preceding bit. A the receiver side, the above process is reversed by comparing a current bit to the preceding bit.
The Phase-Shaped Binary Transmission (PSBT) format in optical communications is a specialized form of Optical Duobinary (ODB) modulation format, where precoded non-return to zero (NRZ) binary waveforms are electrically low pass filtered to correlate adjacent bits with three logical levels and the resultant 3-level electrical signal is used to drive an optical Mach-Zehnder modulator biased at the null of the transfer function to produce the optical PSBT signal. This encoding process results in the encoding of the signal information in both magnitude and phase. Various PSBT optical systems use direct optical detection at the receiver side to convert the received optical PSBT signal into an electrical signal using a photodetector. Optical PSBT has the advantages of narrow optical bandwidth, high chromatic dispersion bandwidth, reduced electrical bandwidth requirement at the optical transmitter side, and a simplified architecture. Narrow band optical filtering of the PSBT signal has been used to obtain a bandwidth-limited PSBT (BL-PSBT) signal and this technique has been shown to improve the optical signal to noise ratio (OSNR) sensitivity of the received signal. This aspect of optical PBST contrasts the reduced performance observed in some other optical modulation formats e.g. NRZ on and off keying (OOK), Return-to-Zero (RZ)-OOK, NRZ-DPSK, and RZ-DPSK.